[Bug target/41514] redundant compare instruction of consecutive conditional branches

[Bug target/41514] redundant compare instruction of consecutive conditional branches

[carrot at google dot com]

------- Comment #1 from carrot at google dot com  2009-09-30 08:25 -------
Created an attachment (id=18671)
 --> (http://gcc.gnu.org/bugzilla/attachment.cgi?id=18671&action=view)
test case


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=41514

[Bug target/41514] New: redundant compare instruction of consecutive conditional branches

[carrot at google dot com]

Compile the attached source code with options -Os -march=armv5te -mthumb, gcc
generates:

        push    {lr}
        cmp     r0, #63     // A
        beq     .L3
        cmp     r0, #63     // B
        bhi     .L4
        cmp     r0, #45
        beq     .L3
        cmp     r0, #47
        bne     .L5
        b       .L3
.L4:
        cmp     r0, #99
        bne     .L5
.L3:
        mov     r0, #1
        b       .L2
.L5:
        mov     r0, #0
.L2:
        @ sp needed for prologue
        pop     {pc}

Instruction B is the same as instruction A, and there are no other instructions
between them clobber condition codes. So we can remove instruction B.


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           Summary: redundant compare instruction of consecutive conditional
                    branches
           Product: gcc
           Version: 4.5.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: carrot at google dot com
 GCC build triplet: i686-linux
  GCC host triplet: i686-linux
GCC target triplet: arm-eabi


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=41514

[Bug target/41514] redundant compare instruction of consecutive conditional branches

[steven at gcc dot gnu dot org]

------- Comment #2 from steven at gcc dot gnu dot org  2009-09-30 08:50 -------
Where does it come from? (Remember: option -dAP, then look at .s file)


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=41514

[Bug target/41514] redundant compare instruction of consecutive conditional branches

[carrot at google dot com]

------- Comment #3 from carrot at google dot com  2009-10-01 07:37 -------
(In reply to comment #2)
> Where does it come from? (Remember: option -dAP, then look at .s file)
> 

The first several instructions and corresponding rtl patterns are:

        cmp     r0, #63    
        beq     .L3
        cmp     r0, #63    
        bhi     .L4

(jump_insn 8 3 35 src/./test5.c:3 (set (pc)
        (if_then_else (eq (reg/v:SI 0 r0 [orig:135 ch ] [135])
                (const_int 63 [0x3f]))
            (label_ref 18)
            (pc))) 201 {*cbranchsi4_insn} (expr_list:REG_BR_PROB (const_int
2900 [0xb54])
        (nil))
 -> 18)

(note 35 8 9 [bb 3] NOTE_INSN_BASIC_BLOCK)

(jump_insn 9 35 36 src/./test5.c:3 (set (pc)
        (if_then_else (gtu (reg/v:SI 0 r0 [orig:135 ch ] [135])
                (const_int 63 [0x3f]))
            (label_ref 14)
            (pc))) 201 {*cbranchsi4_insn} (expr_list:REG_BR_PROB (const_int
5000 [0x1388])
        (nil))
 -> 14)

In thumb's instruction patterns, compare and branch instructions can't be
expressed separately. So we can't easily remove the second compare instruction
in middle end.

I just noticed the second conditional branch (larger than 63) is totally
unnecessary if we compare the equality with 63, 45, 47, 99 one by one. This is
another missed optimization exposed by this test case.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=41514

[Bug target/41514] redundant compare instruction of consecutive conditional branches

[ramana at gcc dot gnu dot org]

-- 

ramana at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
     Ever Confirmed|0                           |1
   Last reconfirmed|0000-00-00 00:00:00         |2009-10-06 07:13:35
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=41514

[Bug target/41514] redundant compare instruction of consecutive conditional branches

[rearnsha at gcc dot gnu dot org]

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rearnsha at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Severity|normal                      |enhancement


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=41514

[Bug target/41514] redundant compare instruction of consecutive conditional branches

[bernds at codesourcery dot com]

------- Comment #4 from bernds at codesourcery dot com  2010-03-31 21:41 -------
A simple patch is at http://gcc.gnu.org/ml/gcc-patches/2010-03/msg01405.html


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bernds at codesourcery dot com changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |bernds at codesourcery dot
                   |                            |com


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=41514

[Bug target/41514] redundant compare instruction of consecutive conditional branches

[bernds at gcc dot gnu dot org]

------- Comment #5 from bernds at gcc dot gnu dot org  2010-04-16 09:42 -------
Subject: Bug 41514

Author: bernds
Date: Fri Apr 16 09:42:32 2010
New Revision: 158404

URL: http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=158404
Log:
        PR target/41514
        * config/arm/arm.md (cbranchsi4_insn): Renamed from "*cbranchsi4_insn".
        If the previous insn is a cbranchsi4_insn with the same arguments,
        omit the compare instruction.

        PR target/41514
        gcc.target/arm/thumb-comparisons.c: New test.

Added:
    trunk/gcc/testsuite/gcc.target/arm/thumb-comparisons.c
Modified:
    trunk/gcc/ChangeLog
    trunk/gcc/config/arm/arm.md
    trunk/gcc/testsuite/ChangeLog


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=41514

[Bug target/41514] redundant compare instruction of consecutive conditional branches

[bernds at gcc dot gnu dot org]

------- Comment #6 from bernds at gcc dot gnu dot org  2010-04-16 11:24 -------
Fixed.


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bernds at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|                            |FIXED


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=41514